Process and apparatus for increasing the size of ammonium sulfate crystals

ABSTRACT

The average size of ammonium sulfate crystals is increased by passing undersized ammonium sulfate crystals through a multistage apparatus in which the crystals are contacted with an aqueous ammonium sulfate spray in a first heated chamber to increase the average size of the crystals, the enlarged crystals thereafter passing through a second heated chamber to dry the enlarged crystals.

This is a continuation-in-part of copending application Ser. No.07/574,205 filed on Aug. 28, 1990 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a process and apparatus for increasing thesize of ammonium sulfate crystals, e.g., from an average size of about0.5 mm to an average size of at least about 1.0 mm. More importantly,this invention relates to the preparation of ammonium sulfate crystalsof uniform size as well as uniform moisture.

The manufacture of ammonium sulfate is an important industrial processand is the subject of a considerable body of patent and non-patentliterature. See, e.g., U.S. Pat. Nos. 1,266,212; 1,919,707; 2,043,067;2,092,073; 2,099,079; 2,102,107; 2,178,082; 2,226,101; 2,228,742;2,368,901; 2,423,794; 2,424,207; 2,599,067; 2,631,084; 2,659,659;2,782,097; 2,795,487; 2,805,125; 2,874,028; and 2,895,800, and Parkash,"Residence Times of Crystals in a Fluidised Bed Crystallizer", Chemistryand Industry, pp. 919-920, Jun. 3, 1967; Parkash et al., "Crystal Growthin Fluidized Beds", Indian Chemical Engineer, pp. 3-6, January 1968;Vragov, "Investigation of Ammonium Sulfate Crystallization in a PseudoLiquified Layer", Sb. Nauch. Tr., Kuzbas. Politekh. Int., No 26, 1970;and, Postnikov et al., "Crystallization of Ammonium Sulfate in aTwo-Circuit Device With a Circulating Suspension", Koksii Khimiya, No.4, pp 24-27, 1983.

It has long been recognized that the tendency of ammonium sulfatecrystals to cake on standing is lessened with increased size of thecrystals. It has therefore been a goal of many of these manufacturingprocesses to provide ammonium sulfate crystals of larger size, e.g.,averaging at least about 1.0 mm. Typically, such increased crystal sizehas been achieved through control of the crystallization process.

Even more significantly, where crystal size is relatively uniform, thetendency of ammonium sulfate to cake on storage is reduced. The reducedtendency to cake is further emphasized when the particles have a uniformmoisture distribution. As used herein, "uniform particle size" meansthat the particles deviate no more than about +10% from the averageparticle size. For example, where the average particle is 1 mm, thesmallest will be 0.9 mm and the largest 1.1 mm.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improved ammoniumsulfate crystals having relatively uniform size and moisture content.

It is another object of the invention to provide a process forincreasing the average size of ammonium sulfate crystals therebylessening the tendency of the crystals to cake during storage.

It is a particular object of the invention to provide such a process,and a suitable apparatus for its practice, in which uniform growth inthe size of ammonium sulfate crystals and subsequent drying of theenlarged crystals is achieved in a multi-stage operation with the feedcrystals descending through the various stages of the operation againsta countercurrent of heated gas, e.g., air.

In keeping with these and other objects, the present invention providesa process for increasing the uniformity size of ammonium sulfatecrystals which comprises:

a) spraying a feed of ammonium sulfate crystals with an aqueous ammoniumsulfate solution such that a substantial number of individual ammoniumsulfate feed crystals are contacted by the ammonium sulfate solution,the contact time of the crystals with the solution and the temperatureof such contact being sufficient to result in uniform ammonium sulfatecrystals; and,

b) heating the ammonium sulfate crystals to reduce their water content.

The invention further provides an ammonium sulfate crystal preparationapparatus which comprises:

a) a first chamber for receiving a feed of ammonium sulfate crystals atone end and discharging enlarged ammonium sulfate crystals of relativelyhigh water content at its other end;

b) means for spraying an aqueous ammonium sulfate solution into thefirst chamber such that a substantial number of individual ammoniumsulfate feed crystals are contacted by the ammonium sulfate solution;

c) means for providing a current of gas in the first chamber which flowscounter to the flow of ammonium sulfate crystals therethrough;

d) means for heating the ammonium sulfate feed crystals in the firstchamber at least when such crystals are contacted with the ammoniumsulfate solution such that the solution undergoes at least partialdrying on the feed crystals resulting in ammonium sulfate crystals ofrelatively uniform size and relatively high water content:

e) a second chamber for receiving the ammonium sulfate crystals ofrelatively high water content discharged from the first chamber at oneend and discharging enlarged, dried ammonium sulfate crystals at theother end;

f) means for providing a current of gas in the second chamber whichflows counter to the flow of ammonium sulfate crystals therethrough;and,

g) means for heating the ammonium sulfate crystals in the second chamberto reduce the water content of the crystals.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic side elevation view of an ammonium sulfatecrystal preparation apparatus in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, ammonium sulfate crystal enlargement apparatus 10 isprovided as a triple-chambered vertical column with ammonium sulfatefeed crystals 11 being introduced to the column from hopper 12 andenlarged anddried ammonium sulfate crystals 13 being discharged from thecolumn at outlet 14. Apparatus 10 includes a first, or spray, chamber 15in which enlargement of the feed crystals takes place. A saturatedaqueous ammoniumsulfate solution is introduced into chamber 15 through aring-shaped spray header 16 which directs the droplets of ammoniumsulfate solution upwardlyagainst descending ammonium sulfate feedcrystals 11 introduced to chamber 15 through inlet 17 controlled byinlet valve 18. The average residency time of the feed crystals inchamber 15 is controlled by suitably regulating the pressure of acountercurrent flow of gas, advantageously air, which is provided byblower 19. The countercurrent flow of gas is maintained in a continuouscircuit, entering the bottom of second, or drying, chamber 20 throughpassage 21, passing upwardly through chambers 20 and 15 and thereafterthrough conduit section 22 where it is driven by blower 19 throughconduit section 23, back into the bottom of second chamber 20, and soforth. The countercurrent flow of gas is heated at any suitable locationalong its circuit and by any suitable means, e.g., in the apparatusshown, by a steam jacket 24 surrounding second chamber 20. Relativelyhigh moisture content gas can be vented from the apparatus as desiredfrom conduit 25 controlled by valve 26 and make-up gas of relatively lowmoisture content can be admitted to the apparatus through conduit 27controlled by valve 28.

As the feed crystals in chamber 15 accumulate additional ammoniumsulfate from the saturated ammonium sulfate solution with which they arecontacted, their apparent density increases to the point where theyovercome the lifting force of the countercurrent gas flow which tends tomaintain the feed crystals in suspension with the chamber, the resultbeing that the by-now enlarged but relatively high moisture contentammonium sulfate crystals descend from chamber 15 through passage 29into second chamber 20 where the countercurrent flow of heated gasretards the descent of the particles for a period of time sufficient todrive off a further quantity of moisture, substantially drying thecrystals. The enlarged and dried ammonium sulfate crystals pass fromchamber 20 through passage 30 (countercurrent flow gas inlet 21) into athird, or accumulator, chamber 31, preferably possessing a conical lowersection 32 in which a mass of enlarged, dried ammonium sulfate crystals13 accumulates. This accumulation of crystals prevents a leakage ofpressurized countercurrent gas from apparatus 10. An ammonium sulfatecrystal level sensor 33 is advantageously provided in chamber 31 tomonitor the level of the accumulated product crystals therein and thusassure the presence of at least an amount of product crystals which willresist or prevent any significant leakage of countercurrent gas from theapparatus. When the mass of accumulated product crystals falls below apredetermined level, as would be the case where the rate of withdrawalof product crystals from chamber 31 exceeds the supply of enlarged,dried crystals thereto from chamber 20, the sensor generates a controlsignal temporarily reducing the speed of product crystal recovery belt34 until the desired level of product crystal in conical section 32 isonce again attained. Similarly, level sensor 33 functions to preventexcessive accumulation of product crystals in chamber 31 by increasingthe speed of belt 34 thus withdrawing product crystals 13 at a fasterrate, until the desired level of the crystals in conical section 32 ofchamber 31 is againachieved. The invention contemplates the use of knownand conventional level sensor units with their attendant signalgenerating, processing and control devices.

The specific operational parameters of the ammonium sulfate crystalenlargement process can be varied widely and are not particularlycritical. Simple and routine experimentation can be used to optimize(maximize) the crystal size for a specific size and shape of the feedcrystals. What is claimed is that the apparatus will produce productcrystals of a large and uniform size. These product crystals not havingany small or undersized members will have a reduced tendency to cake. Inthe case of feed crystals possessing an average size of from about 0.21toabout 0.42 mm, i.e., crystals which are generally regarded to beundersized, the process and apparatus of this invention can readily beoperated to provide product crystals of at least 1.0 mm, and preferablyofat least about 1.3 mm, with a residual moisture content not exceedingabout5 and preferably not exceeding about 1 weight percent. In oneparticular crystal enlargement operation, the velocity of countercurrentgas can be such as to provide a pressure of from about 0.15 to about0.20 and preferably from about 0.16 to about 0.18 psig in first chamber15 and a pressure of from about 0.33 to about 0.39 and preferable fromabout 0.35 to about 0.37 psig in second chamber 20. Such pressures aregenerally sufficient to retard the rate of descent of the ammoniumsulfate crystals in these chambers so that the crystals remain residentin chambers 15 and 20 for appropriate periods of time, i.e., in chamber15 until the feed crystals have increased to a predetermined averagesize and in chamber 20 until the enlarged crystals have been dried to apredetermined maximum level of retained moisture. The temperatureswithin chambers 15 and 20 arelikewise regulated by the heating means,e.g., steam jacket 24, to achieve evaporation of water from the wettedcrystals in chamber 15 and suitable drying of the crystals in chamber 20to provide product crystals 13 in chamber 31. Thus, e.g., temperaturesof the countercurrently flowing gas in chamber 15 can vary from about95° to about 115 ° and preferably from about 100° to about 110° C. andin chamber 19 can vary from about 110° to from about 125° C. andpreferably from about 110° to about 115° C. Depending on thesize ofapparatus 10, throughputs on the order of from about 100 to about 1000and preferably from about 500 to about 700 pounds per hour can bereadily obtained.

What is claimed is:
 1. A process for preparing ammonium sulfate ofuniform size crystals which comprises:a) spraying a feed of descendingammonium sulfate crystals with an aqueous ammonium sulfate solution suchthat individual ammonium sulfate feed crystals are contacted by theammonium sulfate solution; b) providing a countercurrent flow of gashaving a lifting force which retards the rate of descent of the crystalsas they are sprayed, the contact time of the crystals with the solutionand the temperature of such contact being sufficient to enlarge theammonium sulfate crystals until the crystals enlarge to the point wherethey overcome the lifting force of the countercurrent flow of gas; and,c) heating the uniform ammonium sulfate crystals to reduce their watercontent.
 2. The process of claim 1 wherein the ammonium sulfate feedcrystals possess an average size of from about 0.15 to about 0.45 mm andthe average size of the feed crystals is increased to at least 1.0 mm bycontact with the ammonium sulfate solution.
 3. The process of claim 2wherein the average size of the feed crystals is increased to at least1.3 mm.
 4. The process of claim 1 wherein the ammonium sulfate feedcrystals possess an average size of from about 0.20 to about 0.40 andthe average size of the feed crystals is increased to at least 1.0 mm bycontact with the ammonium sulfate solution.
 5. The process of claim 4wherein the average size of the feed crystals is increased to at least1.4 mm.
 6. The process of claim 1 wherein the aqueous ammonium sulfatesolution is a saturated solution.
 7. The process of claim 7 wherein thegas is at a pressure of from about 0.15 to about 0.20 psig.
 8. Theprocess of claim 7 wherein the gas is at a pressure of from about 0.16to about 0.18 psig.
 9. The process of claim 7 wherein the temperature ofthe gas is from about 100° to 110° C.
 10. The process of claim 1 whereinthe ammonium sulfate crystals descend against a countercurrent flow ofgas which retards the rate of descent of the enlarged crystals as saidcrystals are being dried.
 11. The process of claim 10 wherein the gas isat a pressure of from about 0.33 to about 0.39 psig.
 12. The process ofclaim 10 wherein the gas is at a pressure of from about 0.35 to about0.37 psig.
 13. The process of claim 10 wherein the temperature of thegas is from about 110° to about 125° C.
 14. The process of claim 10wherein the temperature of the gas is from about 110° to about 115° C.15. The process of claim 10 wherein the ammonium sulfate crystals aredried to a residual moisture content of not greater than about 5 weightpercent.
 16. The process of claim 10 wherein the ammonium sulfatecrystals are dried to a residual moisture content of not greater thanabout 1 weight percent.